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Will you make your takeoff before you're out of runway? It's not an question you want be asking yourself on your takeoff roll, especially as you approach rotate speed.
So how do you know if you're going to be wheels up before you run out of runway? Obviously, the best place to start is your takeoff performance chart. And after you've run the numbers and determined that you can make a safe takeoff, it's time to back it up with a great rule-of-thumb: the 50/70 rule.
So what is the 50/70 rule? It's a general rule for GA aircraft that says if you haven't reached 70% of your takeoff speed by the time you've reached 50% of the length of the runway, you should abort your takeoff.
Why do you need 70% of your takeoff speed by 50% of the runway? As you accelerate down the runway during takeoff, you start chewing up more feet of runway for every second you're rolling down the pavement. If you haven't achieved 70% of your takeoff speed by the time you're halfway down the runway, you may not have enough pavement left to get to rotation speed and lift off.
So why would that happen, especially if you've already calculated your takeoff distance required?
It could be a number of factors. The winds might have changed prior to takeoff. At some places like mountain airports, it's not uncommon to see the windsock on one end of the runway pointing in a completely different direction than the windsock on the opposite end of the runway. In most GA aircraft, for every 2 knots of tailwind, your takeoff distance increases 10%. So just a few knots of tailwind can make a big difference. It could also be from other factors as well, like an engine problem during takeoff. But whatever the cause, the 50/70 rule will help keep you safe.
Here's how you use it. First, calculate 70% of your takeoff speed. As an example, when we're flying the Cirrus SR-22T, a max gross weight short field takeoff has a rotation speed of 80 knots.
With some quick calculation, you figure that 70% of 80 knots is 56 knots. So that's our target speed: 56 knots.
Next up, you need to find the 50% point on your runway. There are a couple ways you can do that. First, if there's an airport diagram, you can see if there are any taxiways or intersecting runways that are at roughly the 50% point.
If you look at the Steamboat Springs airport, you see that if you're departing runway 32, the first taxiway is at roughly the 50% point of the runway.
So that's a good reference point to use the 50/70 rule. When you start your takeoff from runway 32 at KSBS, you need to achieve 56 knots by the time you reach the first taxiway. If you get to the taxiway and you're not at 56 knots, it's time to abort. And since you can stop your plane much faster than you can accelerate, you'll have enough room to safely stop on the runway.
Another good reference point, which you'll find at larger airports, are runway distance remaining markers. Runway distance remaining markers tell you how many thousands feet of runway are remaining. Here's an example of what they look like.
This example is Aspen, CO, but the signs look the same everywhere: a black sign with white numbers. If you see "2", it means you have 2,000 feet of runway remaining. See a "4"? That means you have 4,000 feet of runway left.
If you're taking off from Aspen, you know that the runway is 8000' long. So, if you use the same takeoff scenario with 56 knots being your target speed at 50% of the runway, you know that you need to achieve 56 knots by the time you reach the "4" sign.
You should always use your takeoff performance charts to make sure you have enough runway for a safe takeoff. But after that, using the 50/70 rule gives you a very good insurance plan when you're rolling down the runway.
If things don't go as planned and you're not getting the performance you expected during takeoff, you'll have a solid "abort takeoff" decision point, with plenty of room to stop.
Like everything in aviation, it's better to make your decision early, instead of waiting until it's too late. If you follow the 50/70 rule, you'll do just that.
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at firstname.lastname@example.org.